1. Field of the Invention
The present invention relates to an optical recording medium in which recording and reproducing of information is performed, and write-once read many is possible by irradiating a laser beam to the optical recording medium to induce optical changes, such as, a light transmittance and a reflectance, to a dye layer and also relates to a method for manufacturing the optical recording medium. The present invention further relates to a method and an apparatus for the recording and reproducing.
2. Description of the Related Art
In recording of information in which reflectance changes associated with a deformation of a dye layer or the like by means of laser beam irradiation is utilized, optical hole burning phenomena induced by optical absorption of a dye layer is used. Write once read many optical recording media that are put into practical use generally recognize a not-recorded portion as in a low-reflectance state and recognize a portion having an optical hole as a record mark in a high-reflectance state. Such a portion having an optical hole comes into a high-reflectance because a metal reflective layer adjacent to the optical hole has a high reflectance.
As an optical recording medium having an organic dye layer as a recording layer, there are many optical recording media known in which a phthalocyanine dye, a cyanine dye, a phenanthrene dye, a naphthoquinone dye, and the like are used.
A read only compact disc recording medium (CD-R) in which an organic dye layer, a metal reflective layer, and a UV curable resin protective layer are sequentially disposed on a substrate requires a high reflectance to meet its CD standards. Therefore, it is needed to develop an organic dye which has a high refraction index at a wavelength of a laser beam for recording and reproducing of information (770 nm to 830 nm) and high performance stability in the recording and reproducing (a wavelength for recording and reproducing of a DVD±R (Digital Versatile Disc±Recordable) is 630 nm to 680 nm).
With respect to CD-R, and DVD±R, there are a number of optical recording media have been proposed in which a cyanine dye and a metal reflective layer, a phthalocyanine dye and a metal reflective layer, or an azo metal-chelate dye and a metal reflective layer or the like are used as recording materials. Examples of the optical recording media include the one that uses a phthalocyanine dye as a recording material (see Japanese Patent Application Laid-Open (JP-A) No. 04-226390), the one that uses an azo metal-chelate dye as a recording material (see JP-A Nos. 04-226390 and 04-46186). However, all of them have only one dye layer and have just a storage capacity of one layer.
Further, to increase a storage capacity of an optical disc, a system having a plurality of data layers has been proposed. An optical disc having tow or more data layers is accessible to various layers by changing its lens focal point.
For example, U.S. Pat. No. 5,202,875 describes an optical disc drive system having a plurality of data layers. The optical disc system comprises any one of a plurality of substrates each of which has data layers with an air-gap spacing provided therebetween, or a plurality of data layers configured to have a solid structure. The optical disc disclosed in U.S. Pat. No. 4,450,553 uses a solid structure in which a plurality of data layers are disposed, and the data layers are respectively a data layer for compact disc (CD).
In addition, optical recording media disclosed in International Publication Nos. W00/016320 and WO00/023990 respectively have a recording layer which comprises only a dye layer or a phase-change layer and do not employ a structure of layers in consideration of its reflectance and compatibility with DVD or the like. For these optical recording media, it is described in their aspects that an information layer comprises two or more layers, but the disclosures do not exactly allow for an optical recording medium having two recording dye layers.
In addition, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-084643 and 2001-10709 disclose an optical recording medium having two inorganic recording layers.
As mentioned above, there are a variety of applications proposed for a multi-layered-structured optical recording medium, but almost all of them were not put into practical use and/or do not allow for its reflectance and compatibility with DVD or the like.
Dye materials have been used since the CD generation, and it is designed to absorb a laser beam at the wavelength to be used. To make an optical recording medium have a multi-layered structure with a dye material, it is required to achieve a balance between a light transmission and a light absorption, but it is technically difficult to do so, and therefore, the development of a technique for making an optical recording medium have a multi-layered structure using a dye material has not yet been completed so far.
In other words, with a conventional structure of an optical recording medium in which a dye layer, a reflective layer, a UV curable resin protective layer are sequentially disposed, it is impossible to give two dye layers to an optical recording medium, because Ag to be used for a reflective layer has a thickness close to 100 nm, and a laser beam hardly transmits to a wavelength of a red laser diode (630 nm to 800 nm) and to a wavelength of a blue laser diode (360 nm to 430 nm).
In Japanese Patent Application Laid-Open (JP-A) No. 2003-331473, a multi-layered optical information recoding medium is proposed, which comprises two or more optical sensory recording layers enabling optically recording and reproducing of information on the substrate and has at least an intermediate layer having a thickness of 0.05 μm to 0.2 μm between the two or more optical sensory recording layers. However, in the proposal a dye layer disposed at the innermost side viewed from the laser beam irradiation side is directly coated on an Ag layer and an Ag alloy layer. Therefore, it still has problems with nonuniformity of dye-coating and portions with short of dye-coating; degradation of jitter properties, which are recording and reproducing properties; and the inclination that an error is liable to occur. Besides, there are still more problems that a reflective layer located at the innermost side from a laser beam irradiation side is subjected to a heavy damage due to its poor heat radiation properties at the innermost layer; deformations of the reflective layer may cause substantial deterioration of groove signals; and tracking of a record mark may be derailed on the side to which a high-recording power is given.
Besides, in Japanese Patent Application Laid-Open (JP-A) No. 2000-311384, an optical recording medium which comprises two layers of light absorptive layer (dye layers) as shown in FIG. 1, is proposed. The proposal is characterized in that two layers of a first light absorptive layer 12 and a second light absorptive layer 16 are disposed therein, and a first barrier layer 13 and a second barrier layer 15 are disposed between the light absorptive layers and an intermediate layer 14. With the structure of layers according to the proposal, however, it has disadvantages that excellent recording and reproducing properties cannot be obtained due to its poor wet properties of dye coating; the reflective layer may be deformed because of its not excellent heat radiation properties brought about by only a reflective layer, which may result in a problem that a tracking trouble or the like may occur.